Method of forming a refractory metal silicide

ABSTRACT

A method of forming a crystalline phase material includes, a) providing a stress inducing material within or operatively adjacent a crystalline material of a first crystalline phase; and b) annealing the crystalline material of the first crystalline phase under conditions effective to transform it to a second crystalline phase. The stress inducing material preferably induces compressive stress within the first crystalline phase during the anneal to the second crystalline phase to lower the required activation energy to produce a more dense second crystalline phase. Example compressive stress inducing layers include SiO 2  and Si 3 N 4 , while example stress inducing materials for providing into layers are Ge, W and Co. Where the compressive stress inducing material is provided on the same side of a wafer over which the crystalline phase material is provided, it is provided to have a thermal coefficient of expansion which is less than the first phase crystalline material. Where the compressive stress inducing material is provided on the opposite side of a wafer over which the crystalline phase material is provided, it is provided to have a thermal coefficient of expansion which is greater than the first phase crystalline material. Example and preferred crystalline phase materials having two phases are refractory metal suicides, such as TiSi x .

This patent resulted from a divisional application of U.S. patentapplication Ser. No. 08/748,997, filed Nov. 14, 1996, entitled “MethodOf Forming A Crystalline Phase Material”, naming Gurtej S. Sandhu andSujit Sharan as inventors, and which is now U.S. Pat. No. 5,997,634, thedisclosure of which is incorporated by reference.

TECHNICAL FIELD

This invention relates generally to formation of crystalline phasematerials in semiconductor wafer processing and more particularly toformation of refractory metal suicides and crystalline phasetransformation thereof.

BACKGROUND OF THE INVENTION

Silicides, such as titanium silicide and tungsten silicide, are commonlyutilized electrically conductive materials in semiconductor waferintegrated circuitry fabrication. Such materials are utilized, forexample, as capping layers over underlying conductively dopedpolysilicon material to form electrically conductive lines orinterconnects. Such silicide materials are also utilized at contactbases intermediate an underlying silicon substrate and overlyingconductive polysilicon contact plugging material. Silicides can beprovided by chemical vapor deposition, or by deposition of elementaltitanium or tungsten over an underlying silicon surface. Subsequent hightemperature annealing causes a chemical reaction of the tungsten ortitanium with the underlying silicon to form the silicide compound.

Titanium silicide (TiSi₂) occurs in two different crystalline structuresor phases referred to as the C49 and C54 phase. The C49 structure isbase-centered orthorhombic, while the C54 is face-centered orthorhombic.The C54 phase occurs in the binary-phase diagram while the C49 phasedoes not. The C49 phase is therefor considered to be metastable. The C54phase is a densely packed structure having 7% less volume than the C49phase. The C54 phase also has lower resistivity (higher conductivity)than the C49 phase.

The C49 phase forms at lower temperatures during a typical refractorymetal silicide formation anneal (i.e. at from 500° C.-600° C.) andtransforms to the C54 phase at higher elevated temperatures (i.e.,greater than or equal to about 650° C.). The formation of the higherresistive C49 phase has been observed to be almost inevitable due to thelower activation energies associated with it (2.1-2.4 eV) which arisesfrom the lower surface energy of the C49 phase compared to that of themore thermodynamically stable C54 phase. Hence, the desired C54 phasecan be obtained by transforming the C49 phase at elevated temperatures.

Due at least in part to its greater conductivity, the C54 phase is muchmore desirable as contact or conductive line cladding material.Continued semiconductive wafer fabrication has achieved denser andsmaller circuitry making silicide layers thinner and narrower in eachsubsequent processing generation. As the silicide layers become thinnerand narrower, the ratio of surface area to volume of material to betransformed from the C49 to the C54 phase increases. This requires everincreasing activation energies to cause the desired transformation,which translates to higher anneal temperatures to effect the desiredphase transformation. In some instances, the temperature must be atleast equal to or greater than 800° C. Unfortunately, heating a silicidelayer to a higher temperature can result in undesired precipitation andagglomeration of silicon in such layer, and also adversely exposes thewafer being processed to undesired and ever increasing thermal exposure.The processing window for achieving or obtaining low resistance silicidephases for smaller line widths and contacts continues to be reduced,making fabrication difficult.

It would be desirable to develop methods which facilitate the C49 to C54phase transformation in titanium silicide films. Although the inventionwas developed with an eye towards overcoming this specific problem, theartisan will appreciate applicability of the invention in other areas,with the invention only being limited by the accompanying claimsappropriately interpreted in accordance with the Doctrine ofEquivalents.

SUMMARY

In but one aspect, the invention provides a method of forming acrystalline phase material. In one implementation, the method isperformed by providing a stress inducing material within or operativelyadjacent a crystalline material of a first crystalline phase prior toanneal. The crystalline material of the first crystalline phase isannealed under conditions effective to transform it to a secondcrystalline phase. The stress inducing material preferably inducescompressive stress within the first crystalline phase during the annealto the second crystalline phase to lower the required activation energyto produce a more dense second crystalline phase.

In accordance another aspect, the invention provides a method of forminga refractory metal silicide. In one implementation, the method isperformed by forming a refractory metal silicide of a first crystallinephase. Compressive stress inducing atoms are provided within therefractory metal silicide of the first crystalline phase, with thecompressive stress inducing atoms being larger than silicon atoms of thesilicide. With the compressive stress inducing atoms within the firstphase refractory metal silicide, the refractory metal silicide of thefirst crystalline phase is annealed under conditions effective totransform said silicide to a more dense second crystalline phase.

In another implementation, a stress inducing material is formed over theopposite side of the wafer over which the first phase crystallinematerial is formed.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention are described below withreference to the following accompanying drawings.

FIG. 1 is a diagrammatic sectional view of a semiconductor waferfragment at one processing step in accordance with the invention.

FIG. 2 is a view of the FIG. 1 wafer at a processing step subsequent tothat shown by FIG. 1.

FIG. 3 is a diagrammatic sectional view of another alternatesemiconductor wafer fragment at an alternate processing step inaccordance with the invention.

FIG. 4 is a view of the FIG. 3 wafer at a processing step subsequent tothat shown by FIG. 3.

FIG. 5 is a diagrammatic sectional view of yet another alternatesemiconductor wafer fragment at another alternate processing step inaccordance with the invention.

FIG. 6 is a view of the FIG. 5 wafer at a processing step subsequent tothat shown by FIG. 5.

FIG. 7 is a diagrammatic sectional view of still another alternatesemiconductor wafer fragment at another alternate processing step inaccordance with the invention.

FIG. 8 is a view of the FIG. 7 wafer at a processing step subsequent tothat shown by FIG. 7.

FIG. 9 is a view of the FIG. 7 wafer at a processing step subsequent tothat shown by FIG. 8.

FIG. 10 is a diagrammatic sectional view of another alternatesemiconductor wafer fragment at another alternate processing step inaccordance with the invention.

FIG. 11 is a view of the FIG. 10 wafer at a processing step subsequentto that shown by FIG. 10.

FIG. 12 is a view of the FIG. 10 wafer at a processing step subsequentto that shown by FIG. 11.

FIG. 13 is a diagrammatic sectional view of another alternatesemiconductor wafer fragment at another alternate processing step inaccordance with the invention.

FIG. 14 is a view of the FIG. 13 wafer at a processing step subsequentto that shown by FIG. 13.

FIG. 15 is a view of the FIG. 13 wafer at a processing step subsequentto that shown by FIG. 14.

FIG. 16 is a view of the FIG. 13 wafer at a processing step subsequentto that shown by FIG. 15.

FIG. 17 is a diagrammatic sectional view of still another alternatesemiconductor wafer fragment at another alternate processing step inaccordance with the invention.

FIG. 18 is a view of the FIG. 17 wafer at a processing step subsequentto that shown by FIG. 17.

FIG. 19 is a view of the FIG. 17 wafer at a processing step subsequentto that shown by FIG. 18.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

This disclosure of the invention is submitted in furtherance of theconstitutional purposes of the U.S. Patent Laws “to promote the progressof science and useful arts” (Article 1, Section 8).

Referring initially to FIGS. 1 and 2, a semiconductor wafer fragment inprocess is indicated generally with reference numeral 10. Such comprisesa substrate 12, for example in the form of a bulk monocrystallinesilicon wafer, having an overlying crystalline material layer 14 capableof undergoing a phase transformation from a first crystalline phase to asecond crystalline phase. Example materials include refractory metalsilicides, such as TiSi_(x) (where “x” ranges from 0.5 to 2.5 and ispredominately “2”) with a first crystalline phase being C49 and a secondcrystalline phase being C54. In the context of this document, the term“semiconductive substrate” is defined to mean any constructioncomprising semiconductive material, including, but not limited to, bulksemiconductive materials such as a semiconductive wafer (either alone orin assemblies comprising other materials thereon), and semiconductivematerial layers (either alone or in assemblies comprising othermaterials). The term “substrate” refers to any supporting structure,including, but not limited to, the semiconductive substrates describedabove.

A layer 16 of compressive stress inducing material is provided over andin contact with (i.e., “on”) first crystalline phase material 14. Layer16 ideally has a thermal coefficient of expansion which is less than thethermal coefficient of expansion of first crystalline phase materiallayer 14, particularly at a desired temperature of phase transformation.Thus, the stress induced in layer 14 at phase transformation anneal willbe of a compressive nature due to the greater expansion properties oflayer 14 as compared to those of layer 16. Layer 16 preferably has athickness which is equal to or greater than a thickness of first phasecrystalline material 14 to facilitate inducing desired stress. Anexample thickness for layers 14 and 16 is from 100 to 2000 Angstroms.Layer 16 is preferably comprised of a material that will not react withthe underlying refractory metal silicide. Example and preferredmaterials for layer 16 include SiO₂ (doped or undoped) and Si₃N₄.

Referring to FIG. 2, first phase crystalline material layer 14 isannealed under conditions effective to transform it to a second moredense and electrically conductive crystalline phase layer 15, such asC54 TiS_(x) in the case of C49 titanium silicide of layer 14. The phasetransformation of a refractory metal silicide from the C49 phase to theC54 phase occurs with the 7% volume reduction or density increase.Compressive stresses induced by the lesser expanding layer 16 duringanneal help to facilitate phase transformation from C49 to C54 by thecompressive forces facilitating this volume reduction, and reduces therequired activation energy for achieving the phase transformation, whichis typically in the prior art provided by temperature anneal alone. Forexample, one prior art processing window for achieving the desired phasetransformation is at a temperature of 800° C. for a tightly controlledperiod of time of from 15-20 seconds for a 350 Angstrom thick C49TiSi_(x) film. Utilizing a compressive stress inducing layer 16 enablestransformation to occur at temperatures less than or equal to about 750°C. in an inert atmosphere (i.e., nitrogen or argon) and with lessstringent time requirements, and thus potentially enables less thermalprocessing of the substrate being treated. An example pressure duringthe anneal would be from 1 Torr to 760 Torr.

The above first described preferred embodiment is but one example of amethod of providing a stress inducing material (i.e., layer 16 )operatively adjacent a crystalline material of a first crystalline phase(i.e. layer 14 ) to be effective to induce stress (i.e. in this examplecompressive stress) as the material is annealed to a second crystallinephase. An alternate example of providing a stress inducing materialoperatively adjacent a crystalline material to be transformed to asecondary crystalline phase is to provide such stress inducing materialunder or inwardly of the first crystalline phase material, as describedwith reference to FIGS. 3-4. Such illustrates a semiconductor waferfragment in process generally with reference numeral 18. In FIG. 3, suchcomprises a substrate 12, for example bulk monocrystalline silicon orlayers of material, having an overlying stress inducing material layer22. A layer 24 of crystalline material of the first crystalline phase isprovided outwardly of layer 22, with layer 22 thus being inwardly of orunder layer 24 and in the illustrated example in contact therewith. Inthe example refractory metal silicide transformation of a C49 phase to aC54 phase accompanied by a volume reduction, layer 22 ideally also has acoefficient of expansion which is less than the coefficient of expansionof layer 24. Such facilitates putting layer 24 in compressive stressduring phase transformation. Example materials include those providedabove for layer 16.

Referring to FIG. 4, annealing is conducted as in the first describedembodiment to transform first crystalline phase material layer 24 into amore dense and higher electrically conductive second phase materiallayer 25.

Yet another alternate example is described with reference to FIGS. 5 and6. Here, the stress inducing material is provided within the crystallinematerial undergoing phase transformation. FIG. 5 illustrates a waferfragment 30 comprised of some substrate construction 32. Again, suchcould be a monocrystalline silicon substrate or some other substrateassembly atop monocrystalline silicon or some other material. Acrystalline material of a first crystalline phase 34, such as arefractory metal silicide, is formed outwardly of substrate 32. Anexample technique, as with the above described embodiment, is bychemical vapor deposition. Alternate examples of providing first phasecrystalline materials for layers 14, 24 and 34 of the first describedembodiments will be described below. Compressive stress inducing atoms36 are provided within first crystalline phase material layer 34. Wherelayer 34 comprises a refractory metal silicide, atoms 36 advantageouslyare provided to be larger than silicon atoms of the silicide to producedesired compressive stress during the anneal to produce the volumereduced phase transformation. Such example atoms include Ge, W and Co ormixtures thereof. One example technique for providing atoms 36 withinlayer 34 is by ion implantation or gas diffusion. An exampleconcentration range is from 10¹⁶-10²² atoms/cm³.

Referring to FIG. 6, the refractory metal silicide of the firstcrystalline phase is annealed under conditions effective to transformsilicide to a more dense second crystalline phase layer 35, with atoms36 inducing compressive stress during such anneal. Anneal conditions asdescribed above are preferred.

Thus, the above described embodiments provide alternate examples ofproviding stress inducing material proximate (either within oroperatively adjacent) a crystalline material of a first crystallinephase which is to undergo phase transformation to a second crystallinephase. In the described and preferred embodiment, such is accompanied bya volume reduction such that the stress induced is desirably of acompressive nature. The above two techniques could of course also becombined such that the stress inducing material is provided both withinand operatively adjacent the material undergoing phase transformation.Further, the stress inducing material layer might be provided prior tothe subject layer being transformed being at the first crystalline phaseconditions. For example, refractory metals when deposited over siliconcontaining layers, such as polysilicon, undergo chemical transformationto silicides merely under elevated temperature anneal conditions. Ineach of the above described embodiments, the stress inducing materialwas provided after the silicide material of the first crystalline phasecame into existence. An alternate example whereby the stress inducingmaterial is provided before the first phase crystalline material comesinto existence is initially described with reference to FIGS. 7-9.

FIG. 7 illustrates a wafer fragment 38 comprised of a substrate in theillustrated form of a silicon, SiO₂ or other material substrate 40having an overlying stress inducing material layer 42, such as SiO₂ orSi₃N₄. An example thickness for layer 42 is from 100-2000 Angstroms. Apolysilicon layer 44 of an example thickness of from 100-2000 Angstromsis provided outwardly of stress inducing material layer 44. Outwardlythereof is provided a refractory metal layer 46, such as elementaltitanium. Thus, a refractory metal (i.e., layer 46) is formed on asilicon containing substrate (i.e. layer 44). The thickness of layer 42is preferably greater than or equal to the combined thickness of layers44 and 46.

Referring to FIG. 8, wafer 38 is annealed to impart a reaction to form arefractory metal silicide layer 48 of, for example, the first C49crystalline phase from the refractory metal of layer 46 and the siliconof the underlying substrate 44. Example anneal conditions include 600°C., 760 Torr in an inert N₂ or Ar ambient for 20 seconds.

Referring to FIG. 9, refractory metal silicide layer 48 of the firstcrystalline phase is annealed to transform the first phase silicide to amore dense second crystalline phase layer 49. Example anneal conditionsfor such phase transformation are as described above with respect to thefirst described embodiments. Alternately, the wafer fragment of FIG. 7could inherently be subjected to the second phase transformation annealconditions at the outset, wherein the wafer being processed wouldinherently be transformed initially to the FIG. 8 embodiment andsubsequently to the FIG. 9 embodiment.

The above described embodiment with respect to FIGS. 7-9 could of coursealso be utilized in conjunction with the FIGS. 5 and 6 embodimentwherein the stress inducing material is provided within the firstcrystalline phase material. For example, the compressive stress inducingatoms can be provided in situ into a refractory metal layer during itsdeposition over an underlying silicon containing substrate. Such couldbe provided for example by sputtering or chemical vapor deposition suchthat the atoms are received within the deposited refractory metal layer.Alternately, ion implanting or gas diffusion doping could be utilized.An example concentration range for the stress inducing atoms is asdescribed above, namely from 10¹⁶-10²² atoms/cm³. Subsequently, therefractory metal layer having the atoms therein would be annealed toform the refractory metal silicide of the first crystalline phase fromthe reaction of the refractory metal and underlying silicon. Continuedor subsequent annealing with the stress inducing atoms in place willfacilitate phase transformation to the second phase.

Another alternate embodiment is described with reference to FIGS. 10-12whereby the stress inducing layer is provided over or outwardly of, andthereby operatively adjacent, the titanium layer prior to its initialtransformation to the first C49 crystalline phase. FIG. 10 illustrates asemiconductor wafer fragment 50 comprised of a bulk monocrystallinesilicon substrate 52 and an overlying insulating layer 54, such as SiO₂.A polysilicon layer 56 is provided outwardly of layer 54, with arefractory metal layer 58, such as titanium, provided outwardly ofpolysilicon layer 56. A compressive stress inducing layer 60 is providedover and on titanium layer 58 and preferably has a thickness equal to orgreater than the combined thickness of layers 56 and 58.

Referring to FIG. 11, suitable annealing conditions for example asdescribed above are utilized to transform layers 56 and 58 into a C49first crystalline phase layer 61.

Referring to FIG. 12, subsequent or continued suitable annealingtransforms first crystalline phase material layer 61 into second C54crystalline phase material layer 63, with the presence of compressivestress inducing layer 60 facilitating such phase transformation asdescribed above.

The above described embodiments can be utilized in contact or any othertechnologies where refractory metal suicides or other crystallinematerials are formed. An example embodiment in utilizing aspects of theabove process in fabricating of electrically conductive lines isdescribed with reference to FIGS. 13-16.

Referring to FIG. 13, a wafer fragment 65 comprises a bulkmonocrystalline silicon substrate 66 having a gate oxide layer 68provided thereover. A layer of polysilicon 70 is provided outwardly ofgate oxide layer 68 with a silicide layer 72 of a C49 first crystallinephase provided outwardly of polysilicon layer 70. Such can be providedby the above or other conventional techniques. Thus, a semiconductivematerial (i.e. silicon of layer 70) is provided over a substrate, (i.e.material 68 and 66), with a refractory metal silicide 72 of a firstcrystalline phase being provided over and in ohmic electrical connectionwith the semiconductive materials. Layer 70 is desirably conductivelydoped with a suitable conductively enhancing impurity either at thispoint or subsequent in the processing.

Referring to FIG. 14, layers 72, 70 and 68 are patterned into conductivelines 74 and 76.

Referring to FIG. 15, a compressive stress inducing material layer 78 isformed outwardly of lines 74 and 76, preferably to a thickness at leastas great as silicide portion 72. Again, preferred materials include SiO₂or Si₃N₄.

Referring to FIG. 16, the wafer fragment is annealed as above totransform the silicide material 72 of the first crystalline phase to C54second crystalline phase material 80. Layer 78 can remain, be removed,anisotropically etched or otherwise processed as the circuitry designdictates.

The above described FIGS. 13-16 embodiment is a technique whereby theconductive line patterning (in this example a gate line) is conductedbefore the annealing, and the compressive stress inducing material isprovided after the line patterning. Alternately, the patterning can beconducted after the annealing. Further, compressive stress inducingmaterial can be provided within the first crystalline phase refractorymetal silicide layer 72 as is for example described with reference tothe FIGS. 5 and 6 embodiment. Alternate techniques are also of coursecontemplated, as will be appreciated by the artisan.

A further alternate embodiment is described with reference to FIGS.17-19. FIG. 17 illustrates a semiconductor wafer fragment 83 (such asmonocrystalline silicon) having opposing first and second sides 84 and85, respectively.

Referring to FIG. 18, a crystalline material layer 87 of a firstcrystalline phase (such as the exemplary C49 TiSi_(x)) is formed overfirst wafer side 84. A compressive stress inducing material layer 89 isprovided over and on second wafer side 85. Layer 89 is provided to havea thermal coefficient of expansion which exceeds that of layer 87. Anexample material where layer 87 comprises a refractory metal silicide isTiN.

Referring to FIG. 19, wafer 83 is annealed under conditions such as thatdescribed above to transform first phase material 87 into second phasematerial 91. The greater coefficient of layer 89 as compared to layer 87causes a degree of bowing which effectively places layer 87 incompressive stress to facilitate its transformation to layer 91.

In compliance with the statute, the invention has been described inlanguage more or less specific as to structural and methodical features.It is to be understood, however, that the invention is not limited tothe specific features shown and described, since the means hereindisclosed comprise preferred forms of putting the invention into effect.The invention is, therefore, claimed in any of its forms ormodifications within the proper scope of the appended claimsappropriately interpreted in accordance with the doctrine ofequivalents.

1. A method of forming a refractory metal silicide comprising: forming acompressive stress inducing material layer over a first side of asubstrate; forming a refractory metal silicide on and in direct physicalcontact with an upper surface of the compressive stress inducingmaterial layer, the refractory metal silicide comprising a firstcrystalline phase; and after forming the refractory metal silicidecomprising the first crystalline phase, annealing the refractory metalcomprising the first crystalline phase at a temperature of less than orequal to about 750° C. to form a refractory metal silicide of a secondcrystalline phase; the compressive stress inducing material inducingsufficient compressive stress to lower an energy of activation fortransformation of the first crystalline phase to the second crystallinephase.
 2. The method of claim 1, where the first crystalline phase isC49 and the second crystalline phase is C54.
 3. The method of claim 1,where the compressive stress inducing material layer comprises siliconoxide or silicon nitride.
 4. The method of claim 1, where the refractorymetal silicide comprises titanium silicide.
 5. The method of claim 4,where the first crystalline phase is C49 and the second crystallinephase is C54.
 6. A method of forming a refractory metal silicidecomprising: forming a compressive stress inducing material layer over afirst side of a substrate; forming a refractory metal silicide on and indirect physical contact with the compressive stress inducing materiallayer, the refractory metal silicide comprising a first crystallinephase; and after forming the refractory metal silicide comprising thefirst crystalline phase, annealing the compressive stress inducingmaterial layer and the refractory metal silicide comprising the firstcrystalline phase to form a refractory metal silicide of a secondcrystalline phase; the compressive stress inducing material inducingsufficient compressive stress to lower en energy of activation fortransformation of the first crystalline chase to the second crystallinephase.
 7. The method of claim 6, wherein forming a compressive stressinducing material layer comprises forming a layer comprising materialschosen from a group consisting of silicon nitride and silicon dioxide.8. The method of claim 6, wherein forming a refractory metal silicidecomprises forming titanium silicide.